Abstract - Paedocypris is a new genus of paedomorphic cyprinid fish from highly acidic blackwater peat swamps in Southeast Asia.

It includes two new species, one of which (Paedocypris progenetica) appears to be the smallest fish and vertebrate known, with the smallest mature female measuring a mere 7.9 mm.

Paedocypris has many 'larval' features typically associated with paedomorphic fish (e.g. narrow frontals that leave the brain unprotected dorsally by bone and a precaudal larval-fin-fold), but, uniquely among fishes, males also possess highly modified pelvic fins with hypertrophied muscles and a keratinized pad in front of the pelvic girdle, which, we hypothesize, function together as a clasping or holding device, thereby suggesting an unusual reproductive mode.

Unfortunately, habitat destruction jeopardizes the survival of these fishes and thus opportunities for further research.

Maurice Kottelat and Peter Ng getting around the problem of viewing the fish one day in the Systematics & Ecology Lab! They had the lab in stitches but luckily Tan Heok Hui took the photo. Thanks to Joelle Lai for remembering!

Scientists from Europe and Singapore say they have discovered the world's tiniest fish -- a species that lives in peat wetlands in Southeast Asia and, when fully grown, is the size of a large mosquito.

The record-busting newcomer to the biodiversity book, Paedocypris progenetica, is a distant cousin of the carp, say the discoverers, who publish their findings on Wednesday in Proceedings of the Royal Society B, a British journal.

Skinny and transparent, the elusive fish lives in highly acid peat swamps on the Indonesian island of Sumatra and in the Malaysian part of Borneo that are threatened by forestry and agriculture.

These so-called "blackwater" swamps are a unique landscape of flooded trees growing in water-logged, soft peaty soil that is often several meters (10 feet) thick.

The water is stained reddish-black, like very dark tea, appearing black at the surface. It is extraordinarily acidic, having a pH value of only three, the same as a sour apple.

The scientists needed a special stereoscopic microscope to accurately measure the fish.

The smallest adult specimen they netted was a mature Paedocypris progenetica female, found in Sumatra, that came to just 7.9mm (0.31 of an inch) from nose to tail, making it not only the world's smallest fish but the smallest vertebrate too.

She nudged out the previous record holder, a marine fish of the Western Pacific called the dwarf goby (Trimmatom nanus), which comes in at 8mm (0.32 of an inch) at sexual maturity.

The team also found a related Paedocypris species, P. micromegethes, in Sarawak in Malaysian Borneo.

At 8.8mm (0.35 of an inch), P. micromegethes is the second smallest freshwater vertebrate ever found.

The fish was discovered by Maurice Kottelat and Tan Heok Hui, who are researchers at the Raffles Museum of Biodiversity Research at the National University of Singapore.

They were assisted by Ralf Britz of Britain's National Museum of Natural History and Kai-Erik Witte at the Max Planck Institute for Developmental Biology in Tuebingen, Germany.

Kottelat said P. progenetica has "a very rudimentary skull" which leaves the brain exposed.

Evolutionary pressures have caused the fish to develop highly modified fins to survive in its special environment. Males also have a tough pad on the front of the pelvic girdle that may be used to help them clutch onto females during mating.

"The discovery of such a tiny and bizarre fish highlights how little we know about the diversity of Southeast Asia," said Kottelat.

"This is all the more serious because the habitat of this fish is disappearing very fast, and the fate of the species is now in doubt."